Lifetime (days)

Figure 5.14 Lifetime of O, with respect to production, loss, and vertical and horizontal transport at the equator, 30°N, and 60°N. Solid lines represent processes that increase Or; dashed lines represent processes that decrease Or (After Ko et at. [190|, Figure 4.)

At all latitudes, above 30 km the lifetime of O, with respect to production and loss is very much shorter than the transport lifetimes. As a result, the distribution of O, is controlled entirely by chemical production and loss. In the lower and mid-stratosphere, the distribution of O, is controlled by a combination of production, loss, and transport—the exact combination is a function of altitude and latitude. This is consistent with the picture presented in Figure 5.3, where it was shown that transport of O, had little impact on the O, distribution above -30 km but plays an important role below -30 km.

It must be remembered that the net of production, loss, and transport in the lower stratosphere is not necessarily zero at any instant in time; instead the balance occurs only when these terms are averaged over a sufficiently long time. Because of seasonal and interannual variations, this "sufficiently long time" can be a year or longer.

Lang-lived tracers Of special interest to the stratospheric chemist are the so-called "long-lived tracers": constituents such as nitrous oxide (N20), methane (CH4), and the CFCs that have long lifetimes (t > years) in the troposphere and lower stratosphere but are destroyed rapidly in the middle and upper stratosphere. We stated earlier (e.g. see Figure 5.12) that the distribution of these species is heavily influenced by transport.

It has been noted that scatter plots of one long-lived tracer against another often show tight one-to-one relationships between the data—even when the species are completely unrelated chemically. Figure 5.15 shows a good example of this tight correlation.

Because these correlations exist, a knowledge of the abundance of one long-lived

CH4 (ppmv)

Figure 5.15 N.O abundance versus CH, abundance, measured between 400 and 450 K potential temperature and between 72.4°S and 68.4°N. Data measured by the shuttle-borne ATMOS instrument [1911 on the ATLAS 1, 2, and 3 missions.

tracer provides information about other long-lived constituents in an air parcel. This is extremely useful when one does not have measurements of all of the constituents needed for an analysis. For example, if one knows that the N,0 in an air parcel is 175 ppbv, then from Figure 5.15 it can be inferred that the CH., abundance is 1.2 ppmv.

To understand why the abundances of long-lived species correlate, one must first understand the concept of slope equilibrium. In regions of the atmosphere where the photochemical lifetime 1/L of a constituent is much longer than the time-scale for horizontal eddy transport (a few months), it was shown by Plumb and Ko [192] that the slopes of the isopleths are functions only of the circulation of the atmosphere [192]. This situation is known as slope equilibrium.

If two species both obey slope equilibrium, isopleths of the species will therefore be coincident, as is shown in the lower stratosphere of Figure 5.16. How one samples this region of the stratosphere is irrelevant [192]. Concentration A1 will always be associated with concentration Bl, A2 with B2, and A3 with B3. A scatter plot of the concentrations will therefore yield a "compact relation": one where the abundance of tracer A is a unique function of the abundance of another tracer B, i.e. [A| =/([B]), with no dependence on other variables such as altitude or latitude. The condition where both constituents obey slope equilibrium is often called the "compact relation regime".

Now, assume that constituent A has a shorter lifetime than constituent B. There is some altitude range in the atmosphere over which constituent A is not in slope equilibrium, but because of its longer lifetime, constituent B is. As a result, the a> .n a.

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